Introduction

There are a variety of applications that securely wipe a disk like shred and dd. Alternatively, dcfldd is an enhanced version of dd with features useful for forensics and security. It accepts most of dd's parameters and includes status output. The last stable version of dcfldd was released on December 19, 2006.[1]

There are three sources of random data commonly used for securely overwriting hard disk partitions; /dev/urandom, badblocks, and shred.

dd and /dev/urandom

#dd if=/dev/urandom of=/dev/<drive> bs=1M

Where /dev/<drive> is the drive to be encrypted.

Note: Using /dev/urandom will take a long time to completely overwrite a drive with "random" data. In the strictest sense, /dev/urandom is less random than /dev/random; however, /dev/random uses the kernel entropy pool and will halt overwriting until more input entropy once this pool has been exhausted. This makes the use of /dev/random for overwriting hard disks impractical.

Note: Users may also find that /dev/urandom takes an excessively long time on large drives of several hundred gigabytes or more (more than twenty-four hours). Frandom offers a faster alternative.

Note: You can retrieve progress of the dd command with this command: kill -USR1 $(pidof dd)

Badblocks

#badblocks -c 10240 -wsvt random /dev/<drive>

Where /dev/<drive> is the drive to be encrypted.

Note: The badblocks command overwrites the drive at a much faster rate by generating data that is not truly random.

shred

From Wikipedia: shred is a Unix command that can be used to securely delete files and devices so that they can be recovered only with great difficulty with specialised hardware, if at all. It is a part of GNU Core Utilities.

shred uses three passes, writing pseudo-random data to the harddrive each pass. This can be reduced or increased.

# shred -v /dev/<drive>

Where /dev/<drive> is the drive to be encrypted. This invokes shred with default settings, displaying the progress to stdout.

# shred --verbose --random-source=/dev/urandom -n1 /dev/<drive>

Invokes shred telling it to only do one pass, with entropy from /dev/urandom. See GNU Coreutils for more information.

Wipe all data left on the device

There may be (possibly unencrypted) data left on the device and you want to protect against simple Forensic Investigation that would be possible with i.e. File Recovery-Software.

If you are not going to set up block device encryption but just want to roughly wipe everything from the disk you could consider using /dev/zero or simple patterns instead of a cryptographically strong random number generator. (Referred to as RNG in this article from now on.) This allows to wipe big disks with maximum performance.

Also read the section on the possibility of #Data remanence if you want to take wiping serious.

Preparations for block device encryption

If you want to prepare your drive to securely set up Block device encryption inside the wiped area afterwards you really should use random data.

Warning: If Block device encryption is mapped on a partition that contains anything else than random/encrypted data, disclosure of usage patterns on the encrypted drive is possible and weakens the encryption the kind of it does for filesystem-level-encryption. Do never use /dev/zero, simple patterns (badblocks, eg.) or other unrandom data before setting up Block device encryption if you are serious about it!

Performance

For Data that is not truely random your disk's writing speed should be the only limiting factor. If you need random data performance may heavily depend on what you choose as source of randomness.

/dev/random

The Kernel built-in RNG /dev/random provides you the same quality random data you would use for keygeneration, but can be nearly impractical to use at least for wiping current HDD capacitys.
What makes disk wiping take so long with is to wait for it to gather enough true entropy.
In an entropy starved situation (e.g. remote server) this might never end while doing search operations on large directories or if your at your desktop running a first person shooter can speed up things a lot.

You can always compare /proc/sys/kernel/random/entropy_avail against /proc/sys/kernel/random/poolsize to keep an eye on your entropy pool.

Tip: The cryptsetup FAQ mentions a very simple procedure to use an existing dm-crypt-Volume to act as a simple PRNG and wipe all free space accesible trough the underlying partition. It is also claimed to protect against disclosure of usage patterns. dd if=/dev/zero of=/dev/mapper/luks Thats it! This will wipe all data written to your dm-crypt Volume.

Conclusion

If you want to wipe sensitive data you can use anything matching your needs.

If you want to setup block device encryption afterwards you should always wipe at least with Pseudorandom data.

As a matter of course the best wiping practice is to never write unencrypted data.

Select a target

Note: Fdisk will not work on GPT formatted devices. Use gdisk instead.

Use fdisk to locate all read/write devices. This will include USB drives if the user can access them. List the partition tables:

# fdisk -l

Check the output for lines that start with devices such as /dev/sda. For example:

Block size

Template:Wikipedia
If you have a Advanced Format hard drive it is recommended that you specify a block size larger than the default 512 bytes. To speed up the overwriting process choose a block size matching your drive's physical geometry by appending the block size option to the dd command (i.e. bs=4096).

To quickly find the block size of the device issue the following command:

Overwrite the disk

Warning: There is no confirmation regarding the sanity of this command so repeatedly check that the correct drive or partition has been targeted. Make certain that the of=... option points to the target drive and not to a system disk.

Zero-fill the disk by writing a zero byte to every addressable location on the disk using the /dev/zero stream.

The process is finished when dcfldd reports, No space left on device. For example:

18944 blocks (75776Mb) written.dcfldd:: No space left on device

Checking progress of dd while running

By default, there is no output of dd until the task has finished. With kill and the "USR1"-Signal you can force status output without actually killing the program. Open up a 2nd root terminal and issue the following command:

# killall -USR1 dd

Note: This will affect all other running dd-processes as well.

Or:

# kill -USR1 <PID_OF_dd_COMMAND>

For example:

# kill -USR1 $(pidof dd)

This causes the terminal in which dd is running to output the progress at the time the command was run. For example:

If the data can be located on the disk and you can confirm that it has never been copied anywhere else, a random number generator provides a quick and thorough alternative.

Residual magnetism

Wiped hard disk drives and other magnetic storage can get disassembled in a cleanroom and then analyzed with equipment like a magnetic force microscope. This may allow the overwritten data to be reconstructed by analyzing the measured residual magnetics.

This method of data recovery for current HDD's is largely theoretical and would require substantial financial resources. Nevertheless degaussing is still practiced.

Old magnetic storage

Securely wiping old magnetic storage (e.g. floppy disks, magnetic tape) is much harder due to much lower memory storage density. Many iterations with random data might be needed to wipe any sensitive data. To ensure that data has been completely erased most resources advise physical destruction.

Filesystem, operation system, programs

The operating system, executing programs or journaling file systems may copy your unencrypted data throughout the block device. However, this should only be relevant in conjunction with one of the above, because you are writing to plain disks.